Is it permissible for us to kill the Khawaarij if we know them and know their wickedness?

هل يجوز قتل الخوارج إذا عرفناهم وعرفنا خبثهم ؟

Answer:

No, if they rebel against the Imaam. 'Alee (radiyallaahu anhu), did not kill them up until they rebelled. The Muslim rulers, even if they are oppressive, (they) do not kill them (khawaarij) up until they draw their swords and rebel (against them), that is if they unsheathe their swords and rebel and kill. If not, then there will be chaos, each one will go and kill the other claiming he is khaarijee.